1. Field of the Invention
The present invention relates to supercalenders of the type wherein the rolls are adapted for limited separation from one another in one mode and are in calendering nipping engagement with one another in another mode.
2. Description of the Prior Art
Supercalenders with which the present invention is concerned are well known in the art and comprise a substantial stack of calender rolls wherein separation of the rolls may be controlled either from the top or the bottom of the stack. Where control is from the bottom of the stack, a king roll at the bottom of the stack is moveable between a lowered position and a raised position. In the lowered position of the king roll, a substantial number of calender rolls thereabove separate from one another to provide gaps therebetween to facilitate threading a new web of material through the stack or to relieve any damaging effect of broke or creased web passing through the roll nips. As thus gapped, the opposite ends of the rolls are supported by their bearing structures on shoulders along upright suspension spindles at the opposite sides of the stack. In the calendering mode of the stack, the king roll, acting through the next adjacent calendering roll, pushes all of the rolls thereabove into nipping relation, wherein the bearing structures for the rolls are lifted from the spindle shoulders. For uniform nip loading, the topmost roll in the stack may be hydraulically biased downwardly. Calenders of the type just described are represented in U.S. Pat. Nos. 3,364,848, 3,369,483, 4,290,351 and 4,311,091.
Inasmuch as the calender rolls are quite heavy, such for instance as about 42,000 pounds each in a supercalender, their bearing structures must be fairly massive to afford adequate support when the rolls are individually supported on the spindles. Typically each bearing structure at each end of each of the rolls may weigh from 4,000 to 5,000 pounds. Therefore, when the rolls are lifted to the nipping, calendering mode, and the bearing structures are in deadweight or overhung weight relation at each end of each roll, the deadweight end loads on the rolls tend to distort the rolls and thus distort the nips between the rolls from the ideal straight line. In other words, ideally the rolls should be ground straight and parallel, without any crown so as to present uniform and straight nips to the paper sheet passing between them, and that relationship should be maintained during the calendering operation.
Heretofore there have been some proposals for relieving bearing deadweight end loads from the rolls. In U.S. Pat. No. 2,985,100, individual load relieving is disclosed as accomplished by means of cables suspended from a frame and carrying stops connected to linking arms, and with pneumatic cylinders connecting the bearing housings with the linking arms which are articulated for the purpose. This patented arrangement is intended only for ordinary paper machine calenders wherein the calender rolls may weigh approximately 10,000 pounds and the bearings about 1,000 pounds each. These weights are only 1/4 to 1/5 of the weights involved in supercalenders. This patented arrangement is unsuited to relieving the dead load of the massive bearing structures of supercalenders.
In British Pat. No. 1,482,379 of 1977, an arrangement is disclosed wherein hydraulic pistons are carried by the nuts mounted on the threaded spindles which are suspended from the cheek plates of the uppermost roll in the stack and which nuts are adapted to effect separation of the rolls upon raising of the top roll. In the nipping mode of the rolls, the hydraulic pistons are adapted to be activated upwardly for relieving the deadweight load of the roll bearings. The hydraulic pistons are also activatable to reduce the entire roll pressure to the extent that the resilient rolls are not damaged during the passage of a joint in the web. When it is desired to separate the nips of the rolls into a gapping relation, it is necessary to deenergize the hydraulic devices, and then when the rolls are returned to the nipping relation if it is desired to effect bearing weight relief, the hydraulic devices must be reenergized.